Tracing the Brain's Reading Network
Excerpt from Science News, 3/7/98

A new study indicates that reading requires the angular gyrus, a brain structure that maintains connections to areas involved with speech comprehension and the integration of alphabet letters with their corresponding sounds.

The reading disorder known as dyslexia may often reflect an inability of the angular gyrus to work in concert with these related brain regions, hold neuroscientist Barry Horowitz of the National Institute on Aging in Bethesda, MD., and his coworkers.

Horowitz's team administered positron emissions tomography, or PET scans to 17 dyslexic men, all of whom had longstanding reading difficulties despite having I.Q.'s in the normal range, and 14 men who read well. Brain imaging was performed as participants read difficult nonsense words (such as "phalbap" in which the "ph" would be pronounced as "f") and real words with unusual pronunciations (such as "choir").

For good readers, these tasks induced simultaneous blood flow surges on the left side of the angular gyrus and in several brain areas that integrate the visual and linguistic information needed for effective reading, the scientists report in the July 21 Proceedings of the National Academy of Sciences.  The men with dyslexia exhibited increased activity in all of these areas except one - the left angular gyrus.

The new findings complement evidence that dyslexia may stem from disturbances in various parts of a brain network that begin working together during childhood as a result of continued exposure to written language, according to the researchers.

(SN: 3/7/98, p. 150).         ---B.B.

Tracking Down the Roots of Dyslexia
---March 1998, Newsweek Magazine

If you are reading these words easily, it's because your brain's visual cortex is seeing the shapes of the letters accurately, and the association cortex is recognizing these marks as letters. A clump of nerve cells called the angular gyrus, toward the back of your brain, is then linking letters with sounds, and the striate cortex farther back is figuring out what the words mean.

But if you are struggling to read, chances are regions in the back of your brain - particularly areas that break down a word into its sounds and put them back together again - are underactive. Or so researchers led by Sally Shaywitz of Yale University found when they compared functional magnetic resonance imaging (fMRI) of the brains of dyslexics with those of nonimpaired readers. Dyslexia is marked by "significant differences in brain activation patterns," concluded the team, reporting its discovery in the Proceeding of the National Academy of Sciences.

The differences in brain activity support the idea that dyslexia stems from a problem translating letters into sounds. They also offer the strongest evidence to date that chronic reading problems, often called a hidden disability because they haven't shown up on X-rays or other tests, arise from real neurological abnormalities.  One day fMRI might be used to diagnose dyslexia early, as every parent of a child at risk hopes. Even more tantalizing, the scans might be used to track what kind of intervention might restore brain activity to normal in children who are slow readers. "If we treat children effectively," says Shaywitz, "it may change their brain organization." Those studies are now underway.

The New England Journal of Medicine - Letters
Dec. 31, 1987 - Dorothy van den Honert

To the Editor: Geiger and Lettvin found that dyslexic subjects recognize letters better when they are presented eccentrically as compared with foveally, and these workers attribute this effect to "an interaction between foveal and peripheral vision that degrades the ability to read in the foveal field." A more parsimonious explanation is available, which also explains various other oddities in dyslexia. The real villain here may well be a defective corpus callosum. Foveal vision involves interhemispheric transfer of information. Peripheral vision goes to one hemisphere only. Perhaps the left hemisphere can learn to deal with direct input from the peripheral vision more easily than it can make sense of double, non-matching input from both sides.

The corpus callosum is suspect in dyslexia in other ways. The only documented visual problems in dyslexia appear in function that require quick and efficient transfer of information across the commissures: smooth convergence, stable ocular dominance, smooth tracking, and matched focusing in the near vision. In addition, persons with dyslexia are poor at tactile localization, which also requires accurate transfer of information across the corpus callosum. The corpus callosum helps control the arousal level of the hemispheres, enabling one to choose which side to use during cognitive tasks. People with dyslexia rely inappropriately on right-brain processing in language handling, which suggests that they have insufficient callosal management.

One very effective way to treat dyslexic persons is to send verbal tasks to one hemisphere while distracting the other with qualitatively different input. This supplies the missing focus and bypasses the corpus callosum and its degraded secondary signals. All this suggests that the time is ripe for a thorough study of the role of the corpus callosum in dyslexia.